1. Field of the Invention
The present invention relates to a driving circuit of a surface light source which is suitable for decreasing the luminance-stabilization period of time and improving the low-temperature starting properties by optimizing a starting voltage and current, and a method of driving the same.
2. Discussion of the Related Art
With the recent development in many kinds of light source, the wide application of the light sources has been accelerated in various fields, for example, illuminating fields, information industrial fields, and image-displaying industrial fields.
The light source is largely classified into a one-dimensional light source including an optical distribution formed in shape of a dot; a two-dimensional light source including an optical distribution formed in shape of a line; and a three-dimensional light source including an optical distribution formed in shape of a surface.
A typical example of the one-dimensional light source corresponds to a light-emitting diode (LED). Also, typical examples of the two-dimensional light source correspond to a cold cathode fluorescent lamp (CCFL) and an external electrode fluorescent lamp (EEFL), and a typical example of the three-dimensional light source corresponds to a flat fluorescent lamp (FFL).
A liquid crystal display (LCD) device necessarily requires an additional backlight since the LCD device is not a self-emission device. For a light source included in the backlight of the LCD device, it is necessary to emit the uniform light in a large-sized area thereof, and to lower the power consumption.
In order to apply the one-dimensional and two-dimensional light sources to the backlight of the LCD device, the light source additionally needs a light-guiding plate (LGP), and optical members including a diffusion member and a prism sheet. Thus, the LCD device using the backlight of the one-dimensional or two-dimensional light source, for example, CCFL or LED, has increased in its volume and weight due to the optical members.
To overcome these problems, a three-dimensional surface light source having a flat type has been newly developed for the backlight of the LCD device. The surface light source may be fabricated with a plurality of discharge sections by forming a glass substrate through the use of a mold or by providing a plurality of glass or ceramic walls between two glass substrates.
The former heats the moldable glass substrate at a predetermined temperature, and then processes the moldable glass substrate by the mold, to thereby form the plurality of discharge sections which are separated from one another by the walls, and are also connected to one another. The processed glass substrate is bonded to another glass substrate by a sealing frit, thereby forming the plurality of discharge sections between the two glass substrates.
The latter forms the plurality of walls using the glass or ceramic material on the glass substrate, and then bonds the glass substrate including the plurality of walls to another glass substrate, thereby forming the plurality of discharge sections between the two glass substrates.
Typically, the FFL of the surface light source uses Hg gas. In comparison to the linear type lamp such as the CCFL or EEFL, the FFL has the larger lamp area and the more channels. Thus, if using the normal driving current and voltage after turning on the FFL, it has the increased time period to stabilize the luminance as compared with that of the related art lamp.
Hereinafter, a related art light source will be explained with the focus on the luminance properties and the low-temperature starting properties.
FIG. 1 is a graph of comparing the luminance-stabilization properties of the two-dimensional light source such as EEFL to the luminance-stabilization properties of the three-dimensional light source such as FFL. FIGS. 2A and 2B are photographs of illustrating the incomplete lighting and the gather of channels on the low-temperature starting and driving mode.
In FIG. 1, (a) illustrates the luminance-stabilization properties of the EEFL, and (b) illustrates the luminance-stabilization properties of the FFL.
Referring to FIG. 1, after starting the EEFL, the EEFL requires the time period of about 5 minutes and 50 seconds to stabilize the luminance thereof. In the meantime, after starting the FFL, the FFL requires the time period of about 18 minutes and 40 seconds to stabilize the luminance thereof. That is, the time period to stabilize the luminance of the FFL is three times as long as the time period to stabilize the luminance of the EEFL. Unless the time period to stabilize the luminance of the FFL becomes shorter, it is difficult to apply the FFL to the backlight of the LCD device.
If the FFL using Hg gas is operated in the low-temperature surroundings, it spends a long time to activate Hg gas. Also, since the flat fluorescent lamp has a large-sized cross section and also includes a plurality of channels, there is high possibility of ununiform discharge.
If the proper voltage and current are not applied to the driving circuit on the low-temperature starting and driving, the incomplete light may occur as shown in FIG. 2A, and the channels may gather to one direction as shown in FIG. 2B. If a winding ratio is increased in primary and secondary windings of a transformer to supply the proper voltage and current (raising the voltage and current), the efficiency of driving circuit is deteriorated.
If the voltage and current are increased to stabilize the initial luminance of driving circuit, it is possible to stabilize the luminance of driving circuit. In this case, unless the voltage and current are slowly decreased by preset periods of time, the flickering and the rapid decrease of luminance may occur.
FIG. 3 is a graph of illustrating the luminance properties if high voltage and current are applied to a flat fluorescent lamp so as to stabilize the luminance. As shown in FIG. 3, if the voltage and current are increased for the initial stabilization of luminance, the luminance is stabilized. However, if maintaining the voltage and current applied to the flat fluorescent lamp, the flickering and the rapid decrease of luminance occur as shown in (A) of FIG. 3.